The present invention relates to data transmission systems and, in particular, to channel coding in such systems.
Much attention has been focused in recent years on channel codes which provide so-called coding gain. Prominent among these are the so-called "trellis" codes described in such papers as G. Ungerboeck, "Channel Coding With Multilevel/Phase Signals," IEEE Trans. Information Theory, IT-28, 1982, pages 55-67; A. R. Calderbank and N. J. A. Sloane, "A New Family of Codes for Dial-Up Voice Lines," Proc. IEEE Global Telecomm. Conf., November 1984, pages 20.2.1-20.2.4; A. R. Calderbank and N. J. A. Sloane, "Four-Dimensional Modulation With an Eight-State Trellis Code," AT&T Technical Journal, Vol. 64, No. 5, May-June 1985, pages 1005-1018; A. R. Calderbank and N. J. A. Sloane, "An Eight-Dimensional Trellis Code," Proc. IEEE, Vol. 74, No. 5, May 1986, pages 757-759; and L.-F. Wei, "Rotationally Invariant Convolutional Channel Coding With Expanded Signal Space--Part I: 180 Degrees and Part II: Nonlinear Codes," IEEE J. Select. Areas Commun., Vol. SAC-2, September 1984, pages 659-686, all of which are hereby incorporated by reference. Commercial use of these codes has, for the most part, been concentrated in voiceband data sets and other carrier data systems. It would, however, be desirable to also use these codes in baseband systems, such as local area networks and telephone interoffice exchange systems. An impediment to the use of trellis codes in baseband systems, however, is the fact that the coded signal generated by conventional trellis coding techniques has a significant component at dc. This is disadvantageous from a number of standpoints. For example, transformer coupling--which is needed, or at least desirable, in many applications--could not be used since transformers will not pass dc. In addition, transmission errors can occur if the remote signal ground reference is not accurate. Moreover, the inclusion of signal power at dc results in a waste of signal power in most detector configurations.